This invention relates to new diisocyanates based on diphenylmethane which may be methyl-substituted and which are characterized, in particular, by isocyanate groups in the 3,4'- and, optionally, in the 3,2'-position. The instant invention also relates to a number of processes for their production and to their use as isocyanate component in the production of polyurethane plastics by the isocyanate polyaddition process.
Among the organic polyisocyanates of commercial and economic significance as starting materials for polyurethane plastics, 2,4-diisocyanatotoluene or mixtures thereof with 2,6-diisocyanatotoluene (TDI) and 4,4'-diisocyanatodiphenylmethane or mixtures thereof with 2,2'- or 2,4'-diisocyanatodiphenylmethane and/or with higher homclogs (MDI) are particularly important. Although these aromatic polyisocyanates are universally used in large quantities for the production of polyurethane plastics, particularly foams and elastomers, their use involves certain disadvantages. Thus, TDI, for example, has an increased vapor pressure in comparison with MDI, which means that, for physiological reasons, appropriate safety measures have to be strictly observed in the processing of this raw material. On the other hand, MDI, i.e. in particular, the 4,4'-diisocyanatodiphenylmethane generally present as main component in polyisocyanate mixtures based on diphenylmethane, is a solid at room temperature with a pronounced tendency towards crystallization. Before processing, these raw materials have to be liquified either by heating to a temperature above the melting point of 4,4'-diisocyanatodiphenylmethane or by chemical modification, for example partial urethane formation (U.S. Pat. No. 3,644,457) or partial carbodiimide formation (U.S. Pat. No. 3,152,162).
The present invention provides new aromatic polyisocyanates which combine the advantages of TDI and MDI without having any of their disadvantages. In addition, the new diisocyanates and diisocyanate mixtures, in contrast to 4,4'-diisocyantodiphenylmethane, contain isocyanate groups of different reactivity which is often of advantage in the production of polyurethane plastics. In addition, the use of the new diisocyanates or diisocyanate mixtures according to the invention in the production of polyurethane plastics makes it possible for new rigid segments corresponding to the hydrocarbon skeleton of the diisocyanates to be incorporated into the polyurethane plastics, which provide a new and interesting possibility for variation in the production of polyurethane plastics.